The proposed project will test the internal and external validity of subtypes of DSM-IV ADHD using multiple converging methods. To accomplish these objectives, measures of ADHD symptoms, functional impairment, intellectual/academic achievement, other psychopathology, and neuropsychological functioning will be administered to the following groups selected from an initial pool of 11,000 non-referred children: 150 probands with the combined subtype, 150 with the hyperactive/impulsive subtype, 150 with the inattentive subtype who also exhibit significant symptoms of sluggish cognitive tempo (SCT), and 150 with the inattentive subtype without significant SCT symptoms, as well as comparison samples of 150 children with significant elevations of SCT who do not meet criteria for any ADHD subtype and 150 children without ADHD or significant SCT symptoms. Reliability and internal validity of the DSM-IV subtypes will be tested by examining inter-rater agreement and conducting exploratory and confirmatory factor and cluster analyses. Longitudinal follow-up measures will be obtained annually to facilitate analyses of the temporal stability and developmental course of the DSM-IV symptoms and subtypes. The diagnostic validity of the DSM-IV subtypes will be examined by testing if each subtype is associated with significant impairment in other domains of functioning. Discriminant validity among the subtypes will be evaluated by comparing subtypes on measures of functional impairment and other clinical correlates, and by testing if symptoms of SCT identify a diagnostically meaningful subset of individuals within the inattentive subtype. To examine both genetic and non-genetic etiological factors, pre/perinatal complications will be assessed and DNA samples will be obtained from all probands, their parents, and the biological sibling closest in age to the proband, facilitating a direct test for differential etiology of ADHD subtypes using genetic linkage and association analyses. Finally, all participants will complete an extensive battery of neurocognitive measures to test for differential performance among the subtypes on cognitive/attentional measures and the contribution of SOT to these differences. It is expected that the use of a large community sample, a longitudinal assessment, an etiologically informative design, and a broad battery of neurocognitive measures will provide important new information regarding the optimal nosology of ADHD.